U.S. Pat. No. 4,378,159 to Galbraith, assigned to the assignee of the present invention, discloses a scanning laser contaminant and defect detector for reflective surfaces having a laser capable of generating a laser beam, a support holding the surface to be inspected, mirrors and other optics for directing and sweeping the beam in a path across the test surface, a light collector, and a light detector for detecting light scattered from the test surface.
In U.S. Pat. No. 4,402,605 to McVay et al., oscillation of a scanning mirror and advance of a plate provides a timing reference to locate particles on the plate. The detector may include a comparator with an adjustable threshold level for generating a signal only when the received scattered radiation exceeds the threshold. The amount of scattering is generally proportional to the size of the particle.
In U.S. Pat. No. 4,482,902 to Bailey et al. a reference clock is used in a scanning system to address memory where pixels identifying the position of particles are recorded. The successive addresses contain pixel location defining codes that compensate for the nonlinearity of a sinusoidal scan rate.
The semiconductor industry uses contaminant detectors to determine the number, size and position of particles on a wafer containing integrated circuits. In this industry, particle detection is particularly important for integrated circuits made using very-large-scale-integration (VLSI) technology. A small number of contaminants can disable a VLSI circuit, so an exact count is necessary to accept or reject wafers and in general provide good quality control.
Unfortunately, laser scanning detectors of the prior art generally overcount the number of particles, because laser beams have a Gaussian intensity distribution and a finite width. While scanning for particles along one path, the "tail" of the beam may cause scattering from particles in adjacent paths. A particle may thus be counted more than once leading to the undesired overcounting. Noise from stray light may also be detected and counted as a particle. Further, when displaying the position of particles with pixels, a pixel is lit when a particle is detected by a beam in that pixel location. Particles near a pixel border may thus light more than one pixel giving inaccurate position information.
An object of the present invention is to devise a method and apparatus for accurately counting the number and determining the position of particles detected by a surface scanner.
Another object of the invention is to devise a method to accurately display the location of particles on the surface, where one particle does not light up more than one pixel.